Thin film gas sensor

Abstract

A gas sensor senses gas using a metal oxide semiconductor whose resistance changes depending on gas adhesion on a surface of the metal oxide semiconductor. The gas sensor includes a base structure, and a gas sensitive layer formed on the base structure and made of a metal oxide semiconductor. The gas sensitive layer has a multi-layer structure which includes at least two layer portions having mutually different grain structures.

What is claimed is: 
     
       1. A gas sensor which senses gas using a metal oxide semiconductor whose resistance changes depending on gas adhesion on a surface of the metal oxide semiconductor, said gas sensor comprising: a base structure; and   a gas sensitive layer formed on said base structure and made of a metal oxide semiconductor,   said gas sensitive layer having a multi-layer structure which includes at least two layer portions having mutually different grain structures.   
     
     
       2. The gas sensor as claimed in claim 1, wherein said gas sensitive layer includes a lower layer portion which has a fine-grain structure and is formed on said base structure, and an upper layer portion which has a continuous layer structure and is formed on the lower layer portion. 
     
     
       3. The gas sensor as claimed in claim 2, wherein said lower layer portion is made up of grains having an average diameter of 0.005 to 5 μm. 
     
     
       4. The gas sensor as claimed in claim 2, wherein said lower layer portion has a thickness which is at least 1/10 of a total thickness of said gas sensitive layer. 
     
     
       5. The gas sensor as claimed in claim 2, wherein said gas sensitive layer is made of a metal oxide material selected from a group consisting of oxides of tin, titanium, indium, nickel, tungsten, cadmium, iron and zinc. 
     
     
       6. The gas sensor as claimed in claim 2, wherein said base structure includes a heater layer and an insulator layer which is formed on the heater layer, said gas sensitive layer being formed on the insulator layer. 
     
     
       7. The gas sensor as claimed in claim 2, wherein said base structure includes a substrate having first and second surfaces and a heater layer formed on the first surface of the substrate, said gas sensitive layer being formed on the second surface of the substrate. 
     
     
       8. The gas sensor as claimed in claim 1, wherein said gas sensitive layer includes a lower layer portion which has a columnar grain structure and is formed on said base structure, an intermediate layer portion which has a fine-grain structure and is formed on the lower layer portion, and an upper layer portion which has a continuous layer structure and is formed on the intermediate layer portion, said columnar grain structure including grains which are arranged approximately perpendicularly to said base structure in the form of columns. 
     
     
       9. The gas sensor as claimed in claim 8, wherein said lower and intermediate layer portions are respectively made up of grains having an average diameter of 0.005 to 5 μm. 
     
     
       10. The gas sensor as claimed in claim 8, wherein said lower layer portion has a thickness which is at least 1/10 of a total thickness of said gas sensitive layer. 
     
     
       11. The gas sensor as claimed in claim 8, wherein said gas sensitive layer is made of a metal oxide material selected from a group consisting of oxides of tin, titanium, indium, nickel, tungsten, cadmium, iron and zinc. 
     
     
       12. The gas sensor as claimed in claim 8, wherein said base structure includes a heater layer and an insulator layer which is formed on the heater layer, said gas sensitive layer being formed on the insulator layer. 
     
     
       13. The gas sensor as claimed in claim 8, wherein said base structure includes a substrate having first and second surfaces and a heater layer formed on the first surface of the substrate, said gas sensitive layer being formed on the second surface of the substrate. 
     
     
       14. The gas sensor as claimed in claim 1, wherein said gas sensitive layer includes a lower layer portion which has a columnar grain structure and is formed on said base structure, and an upper layer portion which has a fine-grain structure and is formed on the lower layer portion, said columnar grain structure including grains which are arranged approximately perpendicularly to said base structure in the form of columns. 
     
     
       15. The gas sensor as claimed in claim 14, wherein said lower and upper layer portions are respectively made up of grains having an average diameter of 0.005 to 5 μm. 
     
     
       16. The gas sensor as claimed in claim 14, wherein said lower layer portion has a thickness which is at least 1/10 of a total thickness of said gas sensitive layer. 
     
     
       17. The gas sensor as claimed in claim 14, wherein said gas sensitive layer is made of a metal oxide material selected from a group consisting of oxides of tin, titanium, indium, nickel, tungsten, cadmium, iron and zinc. 
     
     
       18. The gas sensor as claimed in claim 14, wherein said base structure includes a heater layer and an insulator layer which is formed on the heater layer, said gas sensitive layer being formed on the insulator layer. 
     
     
       19. The gas sensor as claimed in claim 14, wherein said base structure includes a substrate having first and second surfaces and a heater layer formed on the first surface of the substrate, said gas sensitive layer being formed on the second surface of the substrate. 
     
     
       20. The gas sensor as claimed in claim 1, wherein said gas sensitive layer includes a lower layer portion which has a columnar grain structure and is formed on said base structure, and an upper layer portion which has a continuous layer structure and is formed on the lower layer portion, said columnar grain structure including grains which are arranged approximately perpendicularly to said base structure in the form of columns. 
     
     
       21. The gas sensor as claimed in claim 20, wherein said lower layer portion is made up of grains having an average diameter of 0.005 to 5 μm. 
     
     
       22. The gas sensor as claimed in claim 20, wherein said lower layer portion has a thickness which is at least 1/10 of a total thickness of said gas sensitive layer. 
     
     
       23. The gas sensor as claimed in claim 20, wherein said gas sensitive layer is made of a metal oxide material selected from a group consisting of oxides of tin, titanium, indium, nickel, tungsten, cadmium, iron and zinc. 
     
     
       24. The gas sensor as claimed in claim 20, wherein said base structure includes a heater layer and an insulator layer which is formed on the heater layer, said gas sensitive layer being formed on the insulator layer. 
     
     
       25. The gas sensor as claimed in claim 20, wherein said base structure includes a substrate having first and second surfaces and a heater layer formed on the first surface of the substrate, said gas sensitive layer being formed on the second surface of the substrate.